In recent years, from the viewpoint of environmental protection, a power-saving operation has been much needed in a device for driving a motor, such as a fan motor employed for an air conditioner. Under the circumstances, as an effective power-saving technique, an inverter that enables to drive a highly efficient motor, such as a brushless DC motor, at a given frequency has been widely used. Besides, for size reduction, such a driving device employs an IC in which an inverter is integrated to a driver for transmitting driving signals and an overcurrent protection device.
FIG. 5 shows a conventional system structure for driving a motor that employs the aforementioned type of IC. The motor-driving device of FIG. 5 contains IC 52, fixed resistor 54, driving-signal generator 55, inverter 56, driver 57 and first current-protecting section 58. Brushless motor 53 is controlled by IC 52. IC 52 is formed of inverter 56, driver 57 and first current-protecting section 58. Inverter 56 contains three series-circuits each of which has a pair of switching elements arranged on the upstream and on the downstream with respect to the current flow. DC power supply 51 feeds the three series-circuits with DC voltage via fixed resistor 54.
Brushless motor 53 contains three-phase windings that are Y-connected with the neutral point at the center. Driving-signal generator 55 generates PWM signals (i.e., signals for pulse width modulation) to operate brushless motor 53 at an intended speed. Driver 57 amplifies the PWM signals to control inverter 56.
To protect inverter 56 from breakdown caused by overcurrent, first current-protecting section 58 detects voltage across fixed resistor 54 that is connected between the negative side of DC power supply 51 and inverter 56. Reference voltage value Vref1 is preset in first current-protecting section 58. If the detected voltage exceeds reference voltage value Vref1, first current-protecting section 58 shuts off the output of driver 57 and outputs a warning of abnormal conditions. Upon receiving the warning, driving-signal generator 55 stops generating the PWM signals. If once the warning is received, driving-signal generator 55 maintains the shut-off state and generates no more signals.
With the circuit structure above, the motor-driving device controls brushless motor 53. Such a structure described above is disclosed, for example, in Japanese Patent Unexamined Publication No. H09-294392.
In the conventional driving method, however, current-value setting involves following inconveniencies. When the current value of inverter 56 is determined under the assumption that overcurrent occurs once, a continuous flow or a continual flow of current—even when the current value measures lower the predetermined value—often overheats inverter 56, resulting in breakdown. To avoid the overheating, it becomes necessary to use an inverter being more tolerant of heat, or to fix a heatsink with a sufficient capacity to the inverter. This has invited increase in size and cost of the structure. On the other hand, suppose that the current is set at a value in consideration of thermal breakdown caused by the continuous or continual flow of current. In this case, the driving-signal generator 55 becomes sensitive to an overcurrent, which temporarily occurs once or several times in starting-up or accelerating operation of the motor and therefore no risk of thermal breakdown, and shuts off the output. From this reason, the motor has often come to a stop.